The application relates to a bone anchoring device having a bone anchoring element and a rod for connecting at least two bone anchoring elements wherein the rod has at least in a part thereof a tubular structure.
FIG. 12 shows a known polyaxial bone screw 100 with a screw element 101 having a bone thread and a spherical segment-shaped head 102 which is pivotably held in a receiving part 103. The receiving part has a recess for receiving a rod 104. Between the head 102 and the rod a pressure element 105 is provided which is slidable in the receiving part. An inner screw 106 is used to clamp the rod in the receiving part and to exert pressure onto the head via the rod and the pressure element to lock the head. A flexible rod having a tubular structure for stabilizing the spine is known for example from US 2005/0154390 A1. When a tubular rod is used together with the known polyaxial bone screw as described with reference to FIG. 12, the load which acts on the rod when the inner screw is tightened can deform the tubular rod as shown schematically in FIG. 13. This will affect the properties of the rod.
US 2004/0138660 A1 discloses a locking cap assembly for locking a rod which is made from a full metal cylinder to a receiving body of a bone screw. The locking cap assembly includes an inner and an outer locking element. The outer locking element is a nut-like member to which the inner locking element is rotatably connected. The inner locking element has on its side facing the rod a ring-shaped deformable contacting element which comes into contact with the rod. Upon tightening of the outer locking element, the deformable contacting element is deformed which provides feed-back to the surgeon to allow him to determine whether the locking cap assembly is tightened to the required extent.
FR 2 810 533 discloses a bone anchoring device with a rod which is made from a full cylinder. A locking cap assembly comprises a rotatably supported member which presses from above onto the rod. The shape of the rod contacting surface of said member is adapted to the contour of the rod.
Based on the above, there is a need to provide a bone anchoring device which is suitable for a dynamic stabilization and which comprises an improved fixation of the tubular portion of a rod.